“Reasonable Safety” of
Existing Structures, Part 2
Limitations: Buildings and their evaluation
by David G. Tepke, Liying Jiang, Keith E. Kesner, and Stephen S. Szoke
P
art 1 of this series examined some key aspects and considerations when evaluating the safety of existing reinforced concrete structures. Parts 2 and 3 of the series will discuss limitations in evaluating structures and some of the complications presented by the structures themselves. In Part 1, the safety of an existing structure was described as a qualitative concept that only becomes quantitative when viewed probabilistically against predetermined norms or expectations established by a governing body. Effective communication to owners and the public about what licensed design professionals (LDPs) do and do not know, or more correctly, what they can and cannot know, is important. Given the complexity of existingstructures and our evolving understanding of material performance, material variability, and variability in
construction, stating that a structure is safe or unsafe without additional context oversimplifies the issue. Still, it is important to communicate to the public accurate information that is useful and understandable. The following sections describe some of the key concepts and limitations impacting our understanding and ability to make statements regarding safety, with an emphasis on how limited documentation of existing structures and changes in building codes influence our ability to assess existing structures.
Discoverable Documentation
A critical step in evaluating an existing structure is the examination of available information from initial construction and from renovations, maintenance, and repair programs.
For public structures and structures with institutional or professional management, documentation may be extensive.
For many older structures, especially structures that have changed ownership or management, documentation may be limited due to loss or ineffective record keeping. This can lead to a significant gap of knowledge that is difficult or costly to overcome, and one that may present significant uncertainties in the structural evaluation.
The quality of original construction documents can also vary widely, influenced by governing codes and local
practices at the time of construction. In some cases, the construction documents can contain meticulous descriptions of as-built construction, including project specifications, responses to requests for information (RFIs), shop drawings, and construction materials testing records. In other cases, only schematic descriptions of existing construction may be available. When shop drawings, project specifications, and material test reports are not available, it is difficult to ascertain how much change was implemented during construction.
Even when as-built or record drawings are available, there can be questions regarding the completeness and accuracy of these documents.
Information associated with repairs or maintenance sometimes can also be difficult to obtain, as these actions may not be well documented. This includes property condition assessments and structural condition assessment reports that may or may not be preserved, as well as maintenance scopes and repair design documents. Depending on the authority having jurisdiction (AHJ), repair programs may not require permits, which may limit the extent of repair documentation.
When available, documentation and anecdotal information can provide important information for use as a basis, but a level of confirmation commensurate with acceptable risk is necessary when evaluating structures for safety. This is clearly indicated in documents prepared by ACI (that is, ACI
Committees 364, Rehabilitation; and 562, Evaluation, Repair, and Rehabilitation of Concrete Structures) and ASCE/SEI (for example, ASCE/SEI 41-171, Seismic Evaluation and Retrofit of Existing Buildings). The process of verification may result in the identification of conditions that impact safety, including misplaced reinforcing steel, improper design, materials-related distress, or other issues.
Variation in dimensions beyond tolerances permitted in codes and standards may be encountered. This includes improper placement of reinforcing steel resulting in either too little cover, making the structure potentially more susceptible to deterioration, reduced fire rating, or in extreme cases, compromised composite behavior, or too much cover, reducing structural capacity (Fig. 1). Documentation from
or test reports are not available.
However, sampling and testing may be necessary or justified to estimate actual in-place properties if adequate capacity cannot be confirmed through use of the conservative default values.
Tepke3 provides a discussion on the evolution of industry standards and building codes with respect to strength and durability considerations that may impact the performance of existing structures. Knowledge and control of corrosion, alkali-silica reactivity, sulfate attack (internal and external), chemical attack, freezing and thawing, and other deterioration mechanisms have evolved considerably (Fig. 3).
Initiatives such as the durability code being developed by ACI Committee 321, Concrete Durability Code, and the incorporation of global warming potential (GWP) provisions into specifications will modify the way structures are viewed and designed in the future. Structures that predate identification or comprehensive control of a deterioration mechanism may be susceptible to associated damage or deterioration. This must be considered during the assessment stage and factored into consideration for potential safety implications.
Understanding how code provisions have evolved is important in
understanding the safety of an existing structure. The LDP examining existing structures needs to consider both the mechanisms contributing to damage and the susceptibility of the structure as a function of its time of construction.
The LDP must also consider that industry knowledge predates incorporation into industry-standard codes (such as ACI CODE-3184). For example, requirements for the use of air-entraining admixtures to provide resistance to cyclic freezing-and- thawing distress were not implemented until ACI 318-63,5 and comprehensive provisions for air content were not included until ACI 318-71,6 although the need for air entrainment for mitigating freezing-and-thawing damage became understood in the late 1930s. The Bureau of Reclamation required
Fig. 1: Examples of improper placement of reinforcing bars: (a) inadequate cover; and (b) excessive cover
Fig. 2: Evidence of changes in the extent of cracking can impact conclusions developed during an investigation
previous construction litigations, such as complaints and rulings, if available, can provide information regarding defects, particularly cracking (Fig. 2), which is a common topic of dispute. Other conditions that may be encountered include undocumented or unsubstantiated previous removal, addition or alteration of structural elements that could require analysis, or investigation to determine implications. These conditions impact how one may evaluate safety and may be difficult to ascertain from a visual review.
Evolution of Building Codes and the State of Practice
Building codes and standards evolve over time to incorporate knowledge gained through research and practice to provide safer, more durable, and more sustainable structures. Structures constructed to earlier codes inherently lack features identified by newer codes as being important for the intended use.
However, it is recognized that there is
not an expectation of equivalency to new construction. When limited amounts of damage or defects are present, building codes applicable to existing buildings, such as International Existing Building Code (IEBC), International Property Maintenance Code (IPMC), and ACI CODE-562, allow continued use of existing structures. In special cases where an industry-wide issue has been identified that significantly impacts safety, retrofit of existing structures may be required by an AHJ.
ACI CODE-562-212 provides options for using data from original construction drawings, data obtained through sampling and testing, and historical default values provided in the standard as a basis for evaluating concrete and reinforcing steel strength.
Default values in ACI CODE-562-21 are based on conservative estimates of materials typically in use contemporary to their historical periods. This provides an initial check for properties if drawings
(a) (b)
entrained air in concrete as early as the 1940s,7 indicating possible implementation of it in other structures in those early periods. Significant research on corrosion of embedded steel in concrete has been conducted since the 1950s; however, chloride limits for new construction limiting susceptibility to corrosion were not included in the ACI Code until ACI 318-83.8 While delayed ettringite formation (DEF) resulting from excessive curing temperatures was first widely
understood in the 1990s,9 DEF was not addressed in ACI 301 specifications until 2010.10 Other examples of these types of provisions are included in Reference 3.
Other code topics related to safety include the
incorporation of structural integrity provisions for collapse resistance that appeared in ACI 318-8911 and the changes to corrosion protection of prestressing steel that evolved over the last half of the twentieth century.
Further, the LDP should be aware that there is a time lag for incorporation of industry standards into codes adopted by the AHJ, particularly for new codes that require precedence or familiarity prior to inclusion. For example, ACI 562, first published in 2013,12 will be referenced as the revised 2021 version in the 2024 edition of the IEBC, an 11-year lag. There are other situations where there have been lags of 20 years or more of ACI 318 Code requirements being enforced by state or local AHJs. While codes relevant to many older buildings were probably updated less frequently than codes affecting more recent structures, it is important to attempt to track the lineage of codes on existing drawings if available.
Structural systems and their influence on the approach to evaluation
The design of structural systems has evolved over time, with changes made to improve performance and resilience.
The evolution of design means some structural systems are inherently more redundant or are less vulnerable to damage from inadvertent loads, deterioration, design errors, or construction deficiencies. Some considerations for different systems and failure modes are presented in the following sections. However, it is beyond the scope of this article for comprehensive treatment of the systems. ACI PRC-377-2113 provides discussion on collapse prevention of concrete floor systems. Nowak and Szerszen14 and Szerszen and Nowak15 provide discussion on reliability indices for a variety of structural systems, failure modes, and loads for new structures. The application of reliability for determining potentially unsafe conditions in existing structures is
discussed by Stevens et al.16 The following paragraphs discuss some potential vulnerabilities of common structural systems and how industry guides and codes evolved to address them.
Flat plate concrete slabs
Owing to structural efficiency, flat-plate slabs are of relatively thin construction. These systems may include drop panels and capitals at columns to limit deflections and provide additional negative moment capacity and two-way shear
capacity. Due to their thin nature, the placement of reinforcing steel becomes a critical parameter that significantly affects both flexural and two-way (punching) shear capacity.
Punching shear capacity is critical due to its brittle failure mechanism, and thus, conditions that suggest it may be a possible mechanism should be investigated (Fig. 4). The depth of the top reinforcing steel is of critical importance as small variations in steel location can induce large reductions in calculated capacity (for deeper steel) or higher susceptibility to deterioration in corrosive environments (for shallower steel). This is particularly important for structures constructed prior to implementation of provisions for structural integrity.
Load capacity in the vicinity of the column is critical due to possible failures that may initiate sudden collapse. As indicated in ACI PRC-377-21, collapse may be initiated in flat-plate slabs from two-way shear failure of the slab around a column, failure of a column, or flexural failure of the slab.
Settlement or failure of a column effectively sheds loads to
Fig. 3: Freezing-and-thawing/scaling damage in structural slabs from a stadium constructed circa mid 1920s, prior to the use of air- entrainment in concrete
Fig. 4: Cracking at column perimeter that is cause for further investigation (photo courtesy of Structural Technologies)
violent concrete rupture at floors or soffits in occupied space or at slab edges that may create potentially dangerous conditions. Design or construction deficiencies promoting overstress can lead to cracking that impacts overall durability or structural performance (Fig. 5).
Post-tensioned concrete construction requirements and design practices have improved dramatically over the past 70 years, with numerous changes being incrementally added to codes to improve corrosion protection. Post-tensioning used in structures today is required to be sheathed in plastic with corrosion protection and have protected anchorages. Older post-tensioned systems included button head systems, and tendons sheathed in kraft paper. Those systems lacked corrosion protection in the areas behind anchorages and were sensitive to damage. Changes in post-tensioned construction include the development of fully encapsulated strands and more durable sheathing, as described in ACI 423.4-14.17 Prestressed precast concrete
Prestressed concrete components are generally reliant on external connections for connectivity; some that are bearing allow for a moderate amount of movement and others provide restraint. The performance of bearing connections was identified as an issue during the 1994 Northridge earthquake.
Significant changes in design practice were subsequently made to improve diaphragm performance and connections between members.18 The connection between precast double- tee members has also been identified as being susceptible to fatigue and corrosion damage19 (Fig. 6). Improved connection designs are expected to address these issues.
Accessibility, Site Survey Selection, and Subjectivity During Assessment
Evaluation of an existing structure implies some level of examination of the structure to locate possible defects. Some structure types, such as parking structures or Brutalist buildings, provide an open expanse of concrete to examine.
More typical are structures with only limited extents of visible structure due to façades and interior finishes. Removal of finishes, whether interior or exterior, inherently increases the cost and difficulty of a survey. Limitations on accessibility will result in the need for extrapolation of results from the survey locations to the full structure.
In the preceding sections, locations where damage is expected to have a greater effect on the performance of an existing structure are described. Accordingly, when planning an evaluation, these locations are critical spots for probes or other focused investigations. However, these locations may represent only a small fraction of the structure, which introduces sampling subjectivity into the evaluation process.
Consideration of structure globally and particularly sensitive areas locally are important and will be addressed in Part 3 of this series.
Evaluation and analysis of existing structures relies on engineering principles and mechanics. However, conditions
Fig. 5: Cracking and efflorescence in post-tensioned concrete
Fig. 6: A view of a damaged slab soffit at a flange-to-flange connection of precast double tees
the slab connection and to other column/slab connections. As explained in ACI PRC-377-21, progressive or disproportionate collapse can occur from subsequent punching shear failures at surrounding columns, from detachment of slabs that create impact loads on floors below that result in pancaking, and from column failures resulting in additional floor loads. To enhance the resilience of two-way construction, significant changes have been made in design code requirements to provide continuous reinforcement across two-way construction in ACI 318-89.
Unbonded post-tensioned concrete
Unbonded post-tensioned concrete members include tendons that typically extend multiple spans and, therefore, a local failure of a tendon can affect large area in a structure.
The strands are concealed, so it can be difficult to determine the extent of deterioration from corrosion by visual inspection.
Grease stains in slab soffits or deteriorated end anchorage pockets can be indicators of distress, but these conditions may not be prevalent. Additionally, failure of strands can result in
Fig. 7: Ground penetrating radar survey showing locations of stirrups for evaluation of shear capacity
Fig. 8: Horizontal cracking in a beam found to be a manifestation of internal sulfate attack
Fig. 9: Corrosion of embedded reinforcing steel resulting in compromised composite action and development length
encountered in practice often invoke a level of subjectivity. In cases where components of an existing structure are sound and intact, similar methods as those used for new structures can be used to calculate capacity. For example, objective analysis is possible where characteristic strength is different from design, reinforcing steel is improperly located (Fig. 7), or support conditions vary. Conditions of materials-related distress of concrete (Fig. 8) or corrosion of embedded steel (Fig. 9) can result in more uncertainly in the analysis. Bond of reinforcing steel to concrete for supporting conditions of composite action; incremental damage from inherent deterioration from alkali-silica reactivity, DEF, or other mechanisms; impact of corrosion and delamination on bond and development length; and other conditions may not be easily quantifiable due to evaluation or analysis constraints.
The experience of the LDP plays a crucial role in making judgments regarding safety. In some situations, shoring and then implementation of load tests may be prudent; however, where future progressive deterioration is possible from materials-related distress (for example, corrosion),
consideration of future or imminent conditions is necessary in interpretation of load testing result and associated future reduction of capacity from deterioration. Mitigation of the deterioration mechanism through preservation methods and structural health monitoring might be necessary if the structure or component remains in service.
Closing
Part 2 describes some conditions that may impact the evaluation of “safety.” Assessment of existing structures requires an understanding of the expected performance of the structure and how the performance is affected by the
requirements of the building codes in place at the time of construction. The assessment will also be affected by the availability and quality of documentation from original design and building performance over time. The structural system and assessment constraints add additional complexity to the process. Consideration of these parameters in combination with existing conditions may make it difficult to ascertain the overall performance of the structure.
Part 3 of this article series will continue to examine limitations in the evaluation process including historical exposure and use, visual assessment of deterioration and distress, quantification and significance of observed
deterioration, and consideration of the remaining service life.
References
1. ASCE/SEI 41-17, “Seismic Evaluation and Retrofit of Existing Buildings,” American Society of Civil Engineers, Reston, VA, 2017, 623 pp.
2. ACI Committee 562, “Assessment, Repair and Rehabilitation of Existing Concrete Structures Code and Commentary (ACI CODE-562- 21),” American Concrete Institute, Farmington Hills, MI, 2021, 88 pp.
3. Tepke, D.G., “Looking Back to See Ahead—Using Historical Knowledge of Durability to Provide Clues for Concrete Repair,”
Concrete Repair Bulletin, Jan.-Feb. 2023, pp. 10-21.
4. ACI Committee 318, “Building Code Requirements for Structural Concrete and Commentary (ACI CODE-318-19) (Reapproved 2022),”
American Concrete Institute, Farmington Hills, MI, 2019, 624 pp.
5. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-63),” American Concrete Institute, Farmington Hills, MI, 1963, 144 pp.
6. ACI Committee 318, “Building Code Requirements for Reinforced
Concrete (ACI 318-71),” American Concrete Institute, Farmington Hills, MI, 1971, 78 pp.
7. Dolen, T.D., “Historical Development of Durable Concrete for the Bureau of Reclamation,” The Bureau of Reclamation: History Essays from the Centennial Symposium, V. I and II, Bureau of Reclamation, Denver, CO, 2008, pp. 135-151.
8. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-83),” American Concrete Institute, Farmington Hills, MI, 1983, 112 pp.
9. ACI Committee 201, “Durable Concrete—Guide (ACI PRC-201.2-16) (Updated 2023),” American Concrete Institute, Farmington Hills, MI, 2016, 95 pp.
10. ACI Committee 301, “Specifications for Structural Concrete (ACI 301-10),” American Concrete Institute, Farmington Hills, MI, 2010, 64 pp.
11. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-89) and Commentary—ACI 318R-89,”
American Concrete Institute, Farmington Hills, MI, 1989, 353 pp.
12. ACI Committee 562, “Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings (ACI 562-13) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2013, 59 pp.
13. ACI Committee 377, “Integrity and Collapse Resistance of
Structural Concrete Floor Systems—Report (ACI PRC-377-21),”
American Concrete Institute, Farmington Hills, MI, 2021, 24 pp.
14. Nowak, A.S., and Szerszen, M.M., “Calibration of Design Code for Buildings (ACI 318): Part 1—Statistical Models for Resistance,” ACI Structural Journal, V. 100, No. 3, May 2003, pp. 377-382.
15. Szerszen, M.M., and Nowak, A.S., “Calibration of Design Code for Buildings (ACI 318): Part 2—Reliability Analysis and Resistance Factors,” ACI Structural Journal, V. 100, No. 3, May 2003, pp. 383-391.
16. Stevens, G.R.; Bartlett, F.M.; Liu, M.; Kesner, K.E.; and Johnson, G.,
“Evolution of the 562 Code: Quantification of Reliability for Concrete Elements with Demand-Capacity Ratios Greater than One,” Concrete International, V. 41, No. 4, Apr. 2019, pp. 55-61.
17. Joint ACI-ASCE Committee 423, “Report on Corrosion and Repair of Unbonded Single-Strand Tendons (ACI 423.4R-14),” American Concrete Institute, Farmington Hills, MI, 2014, 22 pp.
18. Gould, N.C.; Kallros, M.K.; and Dowty, S.M., “Concrete Parking Structures and the Northridge Earthquake,” Structure Magazine, Oct. 2019, pp. 48-54.
19. Keenan, L.E., “What’s Wrong with My Precast Concrete Garage?”
The Construction Specifier, June 1, 2015, https://www.constructionspecifier.
com/whats-wrong-with-my-precast-concrete-garage/.
Selected for reader interest by the editors.
David G. Tepke, FACI, is a Principal Engineer with SKA Consulting Engineers, Inc., in Charleston, SC, USA. He specializes in structural and materials evaluation, troubleshooting, repair, and service-life extension. He is a NACE/AMPP Certified Corrosion Specialist and Protective Coating Specialist. Tepke is Chair of ACI Committee 222, Corrosion of Metals in Concrete, and a member of the ACI Committee on Codes and Standards Advocacy and Outreach;
and Committees 201, Durability of Concrete; 301, Specifications for Concrete Construction; 321, Durability Code; and 329, Performance Criteria for Ready Mixed Concrete. He is a licensed professional engineer.
ACI member Liying Jiang is an Engineering Manager with Structural Technologies. She specializes in evaluations of existing structures, assessment of concrete materials, design of repair and rehabilitation measures, and development of management strategies for structures affected by alkali-silica reaction (ASR), corrosion, and other materials-related distress. She is Chair of ACI Subcommittee 364-C, TechNote Subcommittee, and Secretary of ACI Subcommittee 228-B, Visual Condition Survey of Concrete.
She is also a member of ACI Committees 228, Nondestructive Testing of Concrete; and 364, Rehabilitation.
Keith E. Kesner, FACI, is a Project Director with Simpson Gumpertz & Heger, Inc.
He specializes in the evaluation and rehabilitation of existing structures. He is Chair of the ACI TAC Repair and Rehabilitation Committee, and ACI Subcommittee 562-E, Seismic. He is also a member of the ACI Committee on Codes and Standards Advocacy and Outreach;
Technical Activities Committee; and Committees 228, Nondestructive Testing of Concrete; 364, Rehabilitation; 562, Evaluation, Repair, and Rehabilitation of Concrete Buildings; and ACI Subcommittees 318-C, Safety, Serviceability, and Analysis; and various state initiatives collaboration groups. He was a co-recipient of the 1998 ACI Construction Practice Award and received the 2005 ACI Young Member Award. Kesner received his BS from the University of Connecticut, Storrs, CT, USA, and his MS and PhD from Cornell University, Ithaca, NY, USA. He is a licensed professional engineer in several states and a licensed structural engineer in Hawaii and Illinois.
Stephen S. Szoke, FACI, ACI Distinguished Staff, is a Code Advocacy Engineer at ACI.
He actively participates in the development of model building codes, referenced standards, rules, and regulations. Szoke is a Staff Liaison for the ACI Committee on Codes and Standards Advocacy and Outreach. He is a licensed professional engineer.
